U.S. patent number 10,144,668 [Application Number 15/504,947] was granted by the patent office on 2018-12-04 for method and apparatus for yielding high edge strength in cutting of flexible thin glass.
This patent grant is currently assigned to Corning Incorporated. The grantee listed for this patent is CORNING INCORPORATED. Invention is credited to Andrew Stephen Altman, Todd Benson Fleming, Anping Liu, James Joseph Watkins.
United States Patent |
10,144,668 |
Altman , et al. |
December 4, 2018 |
Method and apparatus for yielding high edge strength in cutting of
flexible thin glass
Abstract
Methods and apparatus for cutting a glass sheet along a cutting
line into a desired shape. A laser source is configured to apply a
laser beam to a beam location on the cutting line of the glass
sheet. A source of cooling fluid is configured to apply a cooling
fluid to a cooling band on the glass sheet to reduce a temperature
of the glass sheet along the cooling path while elevating the
temperature of the glass sheet at the beam location with the laser
beam. The source of cooling fluid is configured to apply the
cooling path as a cooling ring to circumscribe the beam location on
the cutting line with the cooling band circumferentially spaced
from the beam location while the cooling path and the beam location
move simultaneously together in order to propagate a fracture in
the glass sheet along the cutting line.
Inventors: |
Altman; Andrew Stephen
(Westfield, PA), Fleming; Todd Benson (Elkland, PA), Liu;
Anping (Horseheads, NY), Watkins; James Joseph (Corning,
NY) |
Applicant: |
Name |
City |
State |
Country |
Type |
CORNING INCORPORATED |
Corning |
NY |
US |
|
|
Assignee: |
Corning Incorporated (Corning,
NY)
|
Family
ID: |
54012284 |
Appl.
No.: |
15/504,947 |
Filed: |
August 13, 2015 |
PCT
Filed: |
August 13, 2015 |
PCT No.: |
PCT/US2015/044954 |
371(c)(1),(2),(4) Date: |
February 17, 2017 |
PCT
Pub. No.: |
WO2016/028580 |
PCT
Pub. Date: |
February 25, 2016 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20170275197 A1 |
Sep 28, 2017 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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62039667 |
Aug 20, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C03B
33/091 (20130101); C03B 33/04 (20130101); Y02P
40/57 (20151101) |
Current International
Class: |
C03B
33/04 (20060101); C03B 33/09 (20060101) |
Field of
Search: |
;225/2
;219/121.69,121.68,121.72,121.67 ;359/809 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2008080346 |
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Apr 2008 |
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JP |
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2008127223 |
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Jun 2008 |
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JP |
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2011000308 |
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Jan 2011 |
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KR |
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2012004793 |
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Jan 2012 |
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KR |
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1163394 |
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Jul 2012 |
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KR |
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1195600 |
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Oct 2012 |
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KR |
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2015115604 |
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Aug 2015 |
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WO |
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Other References
Gulati et al; "45.2: Two Point Bending of Thin Glass Substrate";
Society for Information Display (SID) 2011 Digest, p. 652-654.
cited by applicant .
Matthewson et al; "Strength Measurement of Optical Fibers by
Bending"; J. Am. Ceram. Soc., 69 (11), 815-821 (1986). cited by
applicant.
|
Primary Examiner: Alie; Ghassem
Attorney, Agent or Firm: Schmidt; Jeffrey A. Hardee; Ryan
T.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of priority under 35 U.S.C.
.sctn. 371 of International Patent Application Serial No.
PCT/US15/44954, filed on Aug. 13, 2015, which in turn, claims the
benefit of priority of U.S. Provisional Patent Application Ser. No.
62/039,667 filed on Aug. 20, 2014, the contents of each of which
are relied upon and incorporated herein by reference in their
entireties.
Claims
The invention claimed is:
1. A method of propagating a fracture in a glass sheet along a
cutting line of the glass sheet to separate a glass substrate from
the glass sheet, comprising: applying a laser beam to a beam
location on the cutting line of the glass sheet and continuously
moving the beam location relative to the glass sheet along the
cutting line while the laser beam is applied to the beam location
to elevate a temperature of the glass sheet at the beam location on
the cutting line; and applying a cooling fluid to a cooling band on
the glass sheet to reduce a temperature of the glass sheet along
the cooling band while elevating the temperature of the glass sheet
at the beam location with the laser beam, the cooling band
comprises a cooling ring that circumscribes the beam location on
the cutting line with the cooling band circumferentially spaced
from the beam location while the cooling band and the beam location
move simultaneously together in order to propagate a fracture in
the glass sheet along the cutting line to separate the glass
substrate from the glass sheet, and the cooling fluid applies a
mechanical force to the glass sheet sufficient to oppose buckling
of the glass sheet.
2. The method of claim 1, a thickness of the glass sheet is one of:
(i) between about 0.05 mm-about 0.3 mm, and (ii) between about
0.075 mm-about 0.250 mm.
3. The method of claim 2, further compromising controlling the
energy density of the laser to be at least one of: (i) less than
about 0.015 J/mm; (ii) less than about 0.014 J/mm; (iii) less than
about 0.013 J/mm; (iv) less than about 0.012 J/mm; (v) less than
about 0.011 J/mm; (vi) less than about 0.010 J/mm; (vii) less than
about 0.009 J/mm; and (viii) less than about 0.008 J/mm.
4. The method of claim 2, wherein the velocity of the laser beam
relative to the glass sheet along the cutting line is controlled to
be at least one of: (i) less than about 2000 mm/min; (ii) less than
about 1900 mm/min; (iii) less than about 1800 mm/min; (iv) less
than about 1700 mm/min; (v) less than about 1600 mm/min; (vi) less
than about 1500 mm/min; (vii) less than about 1400 min/min; (viii)
less than about 1300 mm/min; (ix) less than about 1200 mm/min; (x)
less than about 1100 mm/min; and (xi) less than about 1000
mm/min.
5. The method of claim 4, wherein the velocity of the laser beam
relative to the glass sheet along the cutting line is .ltoreq.1100
mm/min, and an average number of arrest anomalies is <1 per cut
glass substrate produced, wherein arrest anomalies comprise crack
propagation surging and branching.
6. The method of claim 2, wherein the fluid flow of the cooling
fluid is controlled to be at least one of: (i) at least about 60
lpm; (ii) at least about 70 lpm; (iii) at least about 80 lpm; (iv)
at least about 90 lpm; (v) at least about 100 lpm; (vi) at least
about 110 lpm; (vii) at least about 120 lpm; (viii) at least about
130 lpm; and (ix) at least about 140 lpm.
7. The method of claim 6, wherein the fluid is air.
8. The method of claim 2, wherein the minimum radius of curvature
of the cutting line is controlled to be at least one of: (i) at
least about 2 mm; (ii) at least about 3 mm; (iii) at least about 4
mm; and (iv) at least about 5 mm.
9. The method of claim 1, wherein the cut glass substrate comprises
a probability of failure lower than 10% for a two-point bend test
(B10 edge strength) with an applied load of about 300 MegaPascals
(MPa) to about 500 MPa.
10. The method of claim 9, wherein the cut glass substrate
comprises a probability of failure lower than 10% for a two-point
bend test (B10 edge strength) with an applied load of about 400 MPa
to about 500 MPa.
11. The method of claim 10, wherein the cut glass substrate
comprises a load under which probability of failure is lower than
10% for a two-point bend test (B10 edge strength) with an applied
load of about 450 MPa.
12. The method of claim 1, wherein the laser beam is circular.
13. The method of claim 12, wherein a diameter of the laser beam is
one of: (i) between about 1 mm to about 4 mm, and (ii) 2 mm.
14. An apparatus for cutting a glass sheet along a cutting line to
separate a glass substrate from the glass sheet, comprising: a
laser source configured to apply a laser beam to a beam location on
the cutting line of the glass sheet to elevate a temperature of the
glass sheet at the beam location on the cutting line; and a source
of cooling fluid configured to apply a cooling fluid at about 60
lpm or more to a cooling band on the glass sheet to reduce a
temperature of the glass sheet along the cooling band while
elevating the temperature of the glass sheet at the beam location
with the laser beam, wherein the source of cooling fluid is
configured to apply the cooling band as a cooling ring to
circumscribe the beam location on the cutting line with the cooling
band circumferentially spaced from the beam location while the
cooling path and the beam location move simultaneously together in
order to propagate a fracture in the glass sheet along the cutting
line to separate the glass substrate from the glass sheet.
Description
BACKGROUND
The present disclosure relates to methods and apparatus for
yielding high edge strength in cutting flexible thin glass.
Conventional manufacturing techniques for cutting flexible plastic
substrates have been developed, where the plastic substrates employ
a plastic base material laminated with one or more polymer films.
These laminated structures are commonly used in flexible packaging
associated with photovoltaic (PV) devices, organic light emitting
diodes (OLED), liquid crystal displays (LCD) and patterned thin
film transistor (TFT) electronics, mostly because of their
relatively low cost and demonstrably reliable performance. Although
the aforementioned flexible plastic substrates have come into wide
use, they nevertheless exhibit poor characteristics in connection
with at least providing a moisture barrier and providing very thin
structures (indeed, the structures are relatively thick owing to
the properties of plastic materials).
Accordingly, there are needs in the art for new methods and
apparatus for fabricating a flexible substrate for use in, for
example, PV devices, OLED devices, LCDs, TFT electronics, etc.,
particularly where the substrate is to provide a moisture barrier
and the substrate is to be formed into a free-form shape.
SUMMARY
The present disclosure relates to employing a relatively thin,
flexible, glass sheet (on the order of about 0.05 mm to about 0.3
mm, preferably between about 0.075 mm to about 0.250 mm) and
cutting the glass sheet along a free form line that may include
straight portions as well as curved portions.
Flexible glass substrates offer several technical advantages over
the existing flexible plastic substrate in use today. One technical
advantage is the ability of the glass substrate to serve as good
moisture or gas barrier, which is a primary degradation mechanism
in outdoor applications of electronic devices. Another advantage is
the potential for the flexible glass substrate to reduce the
overall package size (thickness) and weight of a final product
through the reduction or elimination of one or more package
substrate layers. As the demand for thinner, flexible substrates
(of the thickness mentioned herein) increases in the electronic
display industry, manufacturers are facing a number of challenges
for providing suitable flexible substrates.
A significant challenge in fabricating flexible glass substrate for
PV devices, OLED devices, LCDs, TFT electronics, etc., is cutting a
source of relatively large, thin glass sheet into smaller discrete
substrates of various dimensions and shapes with tight dimensional
tolerances, good edge quality, and high edge strength. Indeed, a
desired manufacturing requirement is to cut glass parts off a
source glass sheet continuously, without interruption of the
cutting line, where the cutting line includes at least some round
and/or curved sections (e.g., for rounded corners), possibly of
varying radii, and yielding very good edge quality and strength,
for example on the order of at least about 300 MPa to about 500
MPa, preferably at least about 400-500 MPa, and more preferably at
least about 450 MPa.
Although existing mechanical techniques for continuous cutting of
irregular (free form) shapes provide for scoring (with a score
wheel) and mechanical breaking (or snapping), the edge quality and
strength achieved by such mechanical techniques are not sufficient
for many applications where precision is required. Indeed, the
mechanical scoring and breaking approach generates glass particles
and manufacturing failures, which decreases the process yield and
increases manufacturing cycle time.
In accordance with one or more embodiments herein, a laser cutting
technique is employed to cut a thin glass sheet into a desired
shape. Glass cutting techniques using a laser are known, however,
the cutting of thin flexible glass with thicknesses discussed
herein presents significant challenges, especially when tight
dimensional tolerances and high edge strength are required
manufacturing objectives. The conventional laser score and
mechanical break process is nearly impossible to reliably employ
with glass sheet thicknesses of less than about 0.3 mm, and
especially in the range of 0.05 mm to 0.3 mm. Indeed, due to the
relatively thin profile of a glass sheet of less than about 0.3 mm,
the stiffness of the sheet is very low (i.e., the sheet is
flexible), and the laser score and snap cutting process is easily
adversely affected by thermal buckling, mechanical deformation, air
flows, internal stress, glass warpage, and many other factors.
In contrast, the embodiments herein present laser cutting
techniques resulting in a desired shape of very thin (0.05 mm-0.3
mm) flexible glass, whereby a one-step full separation of the free
form shape from the source glass sheet is achieved along virtually
any trajectory.
The novel methodology and apparatus provides for the propagation of
a crack in the source glass sheet via a laser (for example a CO2
laser beam) and simultaneous provision of a cooling fluid (for
example a gas, for example air). Initiation of the crack is
achieved using a mechanical tool or another laser, for example, and
preferably is disposed outside a perimeter of the desired cutting
line. The methodology and apparatus achieve particularly desirable
results when applied to thin and ultra-thin glass sheets with
thicknesses of less than about 0.3 mm, for example between about
0.05 mm to 0.3 mm, and/or between about 0.075 mm to 0.250 mm.
Notably, cutting of thinner glass sheets is possible, and the
cutting of thicker glass sheets (i.e., greater than about 0.3 mm)
is also possible, although certain parameters may need to be
adjusted in order to still obtain the edge strength results
achieved on glass sheets less than about 0.3 mm. Additionally, the
methodology and/or apparatus achieves an edge quality and strength
of at least about 300 MPa to about 500 MPa, preferably at least
about 400-500 MPa, and more preferably at least about 450 MPa, when
measured by a two point bend test.
Advantages of the embodiments herein include: (i) very good
dimensional stability; (ii) pristine straight cut edges; (iii) low
damage edges on curved edges; (iv) high edge strength; (v) low if
any edge degradation and/or part degradation; (vi) improved
cleanliness; (vii) increased robustness and/or integrity of
finished parts; (viii) meeting and/or exceeding very demanding
customer requirements; (ix) providing a non-contact methodology
that eliminates surface damage from processing.
Other aspects, features, and advantages will be apparent to one
skilled in the art from the description herein taken in conjunction
with the accompanying drawings. For example, the various features
may be combined in any and all combinations as set forth in the
following aspects.
According to a first aspect, there is provided a method,
comprising: supporting a source glass sheet and defining a free
form cutting line that establishes a pattern that defines a desired
final shape; applying a laser beam to the glass sheet and
continuously moving the laser beam relative to the glass sheet
along the cutting line to elevate a temperature of the glass sheet
at the cutting line, where the laser beam is of a circular shape;
applying a cooling fluid simultaneously with the application of the
laser beam, such that the cooling fluid at least reduces the
temperature of the glass sheet in order to propagate a fracture in
the glass sheet along the cutting line; controlling at least one
of: (i) an energy density of the laser beam, (ii) a velocity of the
laser beam relative to the glass sheet along the cutting line,
(iii) a fluid flow of the cooling fluid, and (iv) a minimum radius
of curvature of the cutting line, such that a B10 edge strength of
a cut edge of the glass sheet is at least one of at least about 300
MPa to about 500 MPa, at least about 400-500 MPa, and more
preferably at least about 450 MPa; and separating waste glass from
the glass sheet to obtain the desired shape.
According to a second aspect, there is provided the method of
aspect 1, wherein the glass sheet is one of: (i) between about 0.05
mm-about 0.3 mm in thickness, and (ii) between about 0.075 mm-about
0.250 mm in thickness
According to a third aspect, there is provided the method of aspect
2, wherein the energy density is controlled to be at least one of:
(i) less than about 0.015 J/mm; (ii) less than about 0.014 J/mm;
(iii) less than about 0.013 J/mm; (iv) less than about 0.012 J/mm;
(v) less than about 0.011 J/mm; (vi) less than about 0.010 J/mm;
(vii) less than about 0.009 J/mm; and (viii) less than about 0.008
J/mm.
According to a fourth aspect, there is provided the method of
aspect 3, wherein the energy density is defined to be a power level
of the laser beam (J/s) divided by the velocity (mm/min) of the
laser beam relative to the glass sheet along the cutting line.
According to a fifth aspect, there is provided the method of any
one of aspects 2 to 4, wherein the velocity of the laser beam
relative to the glass sheet along the cutting line is controlled to
be at least one of: (i) less than about 2000 mm/min; (ii) less than
about 1900 mm/min; (iii) less than about 1800 mm/min; (iv) less
than about 1700 mm/min; (v) less than about 1600 mm/min; (vi) less
than about 1500 mm/min; (vii) less than about 1400 mm/min; (viii)
less than about 1300 mm/min; (ix) less than about 1200 mm/min; (x)
less than about 1100 mm/min; and (xi) less than about 1000
mm/min.
According to a sixth aspect, there is provided the method of aspect
5, wherein the velocity of the laser beam relative to the glass
sheet along the cutting line is .ltoreq.1100 mm/in, and the number
of arrest anomalies is <1.
According to a seventh aspect, there is provided the method of any
one of aspects 2 to 4, wherein the fluid flow of the cooling fluid
is controlled to be at least one of (i) at least about 60 lpm; (ii)
at least about 70 lpm; (iii) at least about 80 lpm; (iv) at least
about 90 lpm; (v) at least about 100 lpm; (vi) at least about 110
lpm; (vii) at least about 120 lpm; (viii) at least about 130 lpm;
and (ix) at least about 140 lpm.
According to an eighth aspect, there is provided the method of
aspect 7, wherein the fluid is air.
According to a ninth aspect, there is provided the method of any
one of aspects 2 to 8, wherein the minimum radius of curvature of
the cutting line is controlled to be at least one of: (i) at least
about 2 mm; (ii) at least about 3 mm; (iii) at least about 4 mm;
and (iv) at least about 5 mm.
According to a tenth aspect, there is provided the method of any
one of aspects 1 to 9, wherein a diameter of the laser beam is one
of: (i) between about 1 mm to about 4 mm, and (ii) 2 mm.
According to an eleventh aspect, there is provided the method of
any one of aspects 1 to 10, wherein the cooling fluid is directed
annularly around the laser beam toward the glass sheet.
According to a twelfth aspect, there is provided an apparatus for
cutting a glass sheet into a desired shape, comprising: a support
table operating to support the glass sheet, where the glass sheet
includes a free form cutting line that establishes a pattern that
defines a desired final shape; a laser source operating to apply a
laser beam to the glass sheet that is continuously moved relative
to the glass sheet along the cutting line to elevate a temperature
of the glass sheet at the cutting line, where the laser beam is of
a circular shape; a source of cooling fluid operating to apply a
cooling fluid simultaneously with the application of the laser
beam, such that the cooling fluid at least reduces the temperature
of the glass sheet in order to propagate a fracture in the glass
sheet along the cutting line; a controller operating to control at
least one of: (i) an energy density of the laser beam, (ii) a
velocity of the laser beam relative to the glass sheet along the
cutting line, (iii) a fluid flow of the cooling fluid, and (iv) a
minimum radius of curvature of the cutting line, such that a B10
edge strength of a cut edge of the glass sheet is at least one of
at least about 300 MPa to about 500 MPa, at least about 400-500
MPa, and more preferably at least about 450 MPa.
According to a thirteenth aspect, there is provided the apparatus
of aspect 12, in which the glass sheet is one of: (i) between about
0.05 mm-about 0.3 mm in thickness, and (ii) between about 0.075
mm-about 0.250 mm in thickness.
According to a fourteenth aspect, there is provided the apparatus
of aspect 13, wherein the energy density is controlled to be at
least one of: (i) less than about 0.015 J/mm; (ii) less than about
0.014 J/mm; (iii) less than about 0.013 J/mm; (iv) less than about
0.012 J/mm; (v) less than about 0.011 J/mm; (vi) less than about
0.010 J/mm; (vii) less than about 0.009 J/mm; and (viii) less than
about 0.008 J/mm.
According to a fifteenth aspect, there is provided the apparatus of
aspect 14, wherein the energy density is defined to be a power
level of the laser beam (J/s) divided by the velocity (mm/min) of
the laser beam relative to the glass sheet along the cutting
line.
According to a sixteenth aspect, there is provided the apparatus of
any one of aspects 13 to 15, wherein the velocity of the laser beam
relative to the glass sheet along the cutting line is controlled to
be at least one of: (i) less than about 2000 mm/min; (ii) less than
about 1900 mm/min; (iii) less than about 1800 mm/min; (iv) less
than about 1700 mm/min; (v) less than about 1600 mm/min; (vi) less
than about 1500 mm/min; (vii) less than about 1400 mm/min; (viii)
less than about 1300 mm/min; (ix) less than about 1200 mm/min; (x)
less than about 1100 mm/min; and (xi) less than about 1000
mm/min.
According to a seventeenth aspect, there is provided the apparatus
of aspect 16, wherein the velocity of the laser beam relative to
the glass sheet along the cutting line is controlled to be
.ltoreq.1100 mm/in, and the number of arrest anomalies is
<1.
According to an eighteenth aspect, there is provided the apparatus
of any one of aspects 13 to 17, wherein the fluid flow of the
cooling fluid is controlled to be at least one of (i) at least
about 60 lpm; (ii) at least about 70 lpm; (iii) at least about 80
lpm; (iv) at least about 90 lpm; (v) at least about 100 lpm; (vi)
at least about 110 lpm; (vii) at least about 120 lpm; (viii) at
least about 130 lpm; and (ix) at least about 140 lpm.
According to a nineteenth aspect, there is provided the apparatus
of aspect 18, wherein the fluid is air.
According to a twentieth aspect, there is provided the apparatus of
any one of aspects 13 to 19, wherein the minimum radius of
curvature of the cutting line is controlled to be at least one of:
(i) at least about 2 mm; (ii) at least about 3 mm; (iii) at least
about 4 mm; and (iv) at least about 5 mm.
According to a twenty first aspect, there is provided the apparatus
of any one of aspects 12 to 20, wherein a diameter of the laser
beam is one of: (i) between about 1 mm to about 4 mm, and (ii) 2
mm.
According to a twenty second aspect, there is provided the
apparatus of any one of aspects 12 to 20, wherein the cooling fluid
is directed annularly around the laser beam toward the glass
sheet.
DESCRIPTION OF THE DRAWINGS
For the purposes of illustration, there are forms shown in the
drawings that are presently preferred, it being understood,
however, that the embodiments disclosed and described herein are
not limited to the precise arrangements and instrumentalities
shown.
FIG. 1 is top view of a thin, glass substrate produced using one or
more cutting methodologies and apparatus disclosed herein;
FIG. 2 is a top view of a source glass sheet from which the glass
substrate of FIG. 1 may be produced;
FIG. 3 is a schematic illustration of an apparatus that may be used
to cut the glass substrate from the glass sheet;
FIG. 4 is a plot of a relationship between energy density of the
laser beam and compression hackle in the glass substrate;
FIG. 5 is a plot of a relationship between velocity of the laser
beam and compression hackle in the glass substrate;
FIG. 6 is a plot of the relationship between the flow rate of the
cooling fluid and the resultant compressive hackle in the glass
substrate;
FIG. 7 is a localized side view of the mechanical effects of the
cooling fluid applied from above the glass sheet;
FIG. 8 is a plot illustrating the relationship of a radius of the
cutting line and the resultant compressive hackle in the glass
substrate;
FIG. 9 is a plot showing edge strength of glass substrates cut
using the laser process, particularly failure probability versus
maximum stress; and
FIG. 10 is a plot showing edge strength of glass substrates cut
using a mechanical process, particularly failure probability versus
maximum stress.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference to the drawings wherein like numerals indicate like
elements there is shown in FIG. 1 a top view of a thin, glass
substrate 10 produced using one or more cutting methodologies and
apparatus disclosed herein. A number of characteristics of the
glass substrate 10 are of importance when considering the
disclosure herein. First, the glass substrate 10 (and the source
glass sheet from which it is cut) is thin and/or ultra-thin, with a
thickness of: (i) between about 0.05 mm-about 0.3 mm, and (ii)
between about 0.075 mm-about 0.250 mm. While the advantageous edge
characteristics, including high strengths and minimization of edge
imperfections associated with the crack propagation, discussed
herein are achieved with these thicknesses, the glass substrate 10
may be thinner and/or thicker than the ranges mentioned and
desirable results still obtained although some of the parameters
discussed (energy density, cutting velocity, cooling fluid flow,
and/or radius of curvature) may need to be adjusted with other
thicknesses and/or compositions of glasses. Second, the glass
substrate 10 may be considered a free form shape, for example
having at least one curved portion, and indeed potentially a
plurality of curved portions, having one or more radii of
curvature. For example, the glass substrate 10 is shown with four
rounded corners, although any other shape may be employed, for
example having a mix of rounded corners, sharp corners, straight
beveled corners, etc. Third, the glass substrate 10 is intended to
be formed via a one step, full separation cutting methodology in
which the desired shape is obtained from a thin source glass
sheet.
Another characteristic of the glass substrate 10, which is singled
out for further discussion, is the quality and strength of the cut
edges. In particular, the edge strength is at least one of: at
least about 300 MPa to about 500 MPa, at least about 400-500 MPa,
and more preferably at least about 450 MPa, when the substrate is
subjected to a two point bend test. The two point bend test was
carried out as described in the Society for Information Display
(SID) 2011 Digest, pages 652-654, in a paper entitled "Two Point
Bending of Thin Glass Substrate" by Suresh Gulati, et al. Such a
high edge strength in combination with the relatively thin glass
substrate 10 has not heretofore been achieved.
Reference is now made to FIGS. 2-3, which show a top view of a
source glass sheet 20 from which the glass substrate 10 of FIG. 1
may be produced, and a schematic illustration of an apparatus that
may be used to cut the glass substrate 10 from the glass sheet 20,
respectively. The novel methodology and apparatus disclosed herein
provides for cutting the glass substrate 10 via propagation of a
crack in the source glass sheet using a laser (for example a CO2
laser beam) and simultaneous provision of a cooling fluid (for
example, a gas, such as air). In general, this arrangement results
in the controlled propagation of the crack in the source glass
sheet 20 along the desired cutting line in order to separate the
glass substrate 10 from the glass sheet 20. A more detailed
discussion of the methodology and apparatus for carrying out the
initiation, propagation, and termination of the crack is provided
later in this description.
As an initial phase of the process, the source glass sheet 20 (of
the aforementioned thickness) is supported on a suitable support
structure 102 and a free form cutting line (the dashed line in FIG.
2) is defined that establishes a closed pattern, where the cutting
line circumscribes the desired final shape of the glass substrate
10. The support structure 102 may be configured to permit movement
of the glass sheet 20 into position for laser cutting, and then
movement of the glass substrate 10 (after the cutting process is
complete) for further processing. For such purposes, the support
structure 102 may include an air bearing mechanism, such as a
commercially available pressure/vacuum table with discrete air and
vacuum holes. The support structure 102 may provide: (i) an
air-bearing mode for transportation of the glass sheet 20 and the
glass substrate 10, and (ii) a combined air-bearing and vacuum
mode, for holding the glass sheet 20, during laser cutting.
The air bearing mode is characterized by applying support fluid to
one or more respective portions of the glass sheet 20 at least in
proximity to the cutting line but preferable over a much larger
area, and from a side (the underside) of the glass sheet 20
opposite to the cooling fluid 62 and the laser beam 60. The support
fluid of the air bearing is delivered from the surface of the
support structure 102 by way of the porosity of the surface and a
source of fluid of varying pressure and flow (not shown). The air
bearing mode operates to bias the glass sheet 20 away from the
surface of the table of the support structure 20 as the laser beam
60 elevates the temperature of the glass sheet 20 and the cooling
fluid 62 is directed in opposing fashion to the support fluid. The
air bearing and vacuum mode provides for both positive fluid
pressure (discussed above) and negative fluid pressure and flow to
the glass sheet 20, thereby biasing and holding the glass sheet 20
toward the surface of the support structure 102 at a particular fly
height.
An initial crack is initiated over a small length on the glass
sheet 20, which is subsequently propagated using the aforementioned
laser cutting technique. In general, the glass sheet 20 is scored
at an initiation line (the initial crack) using a mechanical
scoring device, for example a score wheel. Alternatively, a laser
may be used to form an initiation crack or flaw to be propagated by
the laser technique described. In order to appreciate the
significance of the crack initiation and subsequent propagation of
the crack, a more detailed discussion of the laser cutting
technique will first be provided.
The laser beam 60 is used to heat the glass sheet 20 in a localized
area and then to rapidly cool that area via the cooling fluid 62 in
order to create transient tensile stress via the resultant
temperature gradient. The aforementioned initial crack (initiation
line) is created by introducing a small initial flaw on the surface
of the glass sheet 20, which is then transformed into a vent (the
crack) propagated by heating the localized zone via the laser beam
60 and cooling that zone via quenching action produced by the
cooling fluid 62. The tensile stress, .sigma., produced during the
process is proportional to .alpha.*E*.DELTA.T, where a is a linear
thermal expansion coefficient of the glass sheet 20, E is a modulus
of elasticity of the glass sheet 20, and .DELTA.T is a temperature
difference on the surface of the glass sheet 20 produced by the
heating (from the laser) and the cooling (from the fluid). The
tensile stress is controlled in order to be higher than the
molecular bonds of the glass sheet 20. For a given .alpha.*E
tensile stress, .sigma. can be increased by heating the glass sheet
20 to a higher temperature via the laser beam 60. The described
method uses full body glass separation (cutting), where the vent
depth is equal to the thickness of the glass sheet 20.
The laser beam 60 may be implemented using a source 64 of laser
energy, folding optics 66, and focusing optics 68. Application of
the laser beam 60 to the glass sheet 20 starting at the initiation
line (the initial crack) initiates propagation of the crack.
Continuous moving of the laser beam 60 relative to the glass sheet
20 along the cutting line elevates the temperature of the glass
sheet 20 at the cutting line (preferably to a substantially
consistent temperature). Simultaneously, the cooling fluid 62 is
applied relative to the laser beam 60 (via a nozzle 70), such that
the cooling fluid 62 causes a temperature differential in the glass
sheet 20 in order to induce the aforementioned tensile stress and
propagate the crack (i.e., a fracture or vent) in the glass sheet
20 along the cutting line. Movement of the laser beam 60 and nozzle
70 relative to the glass sheet 20 may be achieved through any of
the known conveyance mechanisms, including those wherein the laser
and nozzle are moved relative to a stationary sheet, those wherein
a sheet is moved relative to a stationary laser and nozzle, or
those having a combination of both of the aforementioned conveyance
mechanisms.
A particular combination of laser beam size, laser beam shape, and
cooling fluid delivery affects the crack initiation, propagation,
and termination in favorable ways. To appreciate the contemplated
combination, a brief discussion of a traditional laser beam
configuration is provided. In particular, the traditional
configuration includes an elongated laser beam of various
dimensions followed by the cooling fluid--where the source of the
cooling fluid is positioned in an offset linear relationship (a
trailing configuration) with respect to the elongate laser beam.
This traditional arrangement (elongate laser beam and trailing
coolant) is very efficient for straight laser cutting (or scoring),
however, it does not allow for changing the direction of the crack
propagation--and therefore no curved crack propagation is possible.
Curved, free form, laser cutting may be achieved using the laser
beam 60 of a round shape surrounded by an annular, circular,
ring-shaped coolant fluid 62 (achieved using the coolant source
nozzle 70). The circular laser beam 60, together with the annular
coolant zone 62 does not exhibit any predefined or inherent
orientation, and therefore can be used to propagate the crack in
any direction (without having to use any complex beam shaping
techniques or provide any additional motion axes for movement of
the nozzle 70).
The source of laser power 64 may be implemented using CO2 laser
mechanisms, however, other implementations are possible, for
example a fiber laser, an Nd:YAG laser, or other laser systems. The
type of laser system used may be matched with the composition and
characteristics of the material to be cut. As long as the laser can
induce sufficient thermal stress to overcome prevalent stresses in
the substrate (which stresses in the substrate may be naturally
present, or may be created by chemical or thermal strengthening,
for example), it can cut the substrate. A carbon dioxide laser
operates at the wavelength of 10.6 .mu.m, which works well for
display-type glasses, for example alkali-free alumino-boro silicate
glasses, for example Corning.RTM. code EAGLE XG.RTM. glass
available from Corning Incorporated, Corning N.Y. In general, using
a laser beam 60 having the diameters disclosed herein allows
certain advantageous effects, such as minimization of edge
imperfections associated with the crack propagation (the smaller
the beam diameter, the smaller the unstable crack propagation
zone), and maintaining a reasonably high cutting speed even with a
small diameter beam, resulting in relatively short processing time
and high throughput.
With reference to FIGS. 4-7, it has been found that controlling one
or more of a number of processing parameters during the cutting
process has an appreciable effect on the quality of the cut edges
of the resulting glass substrate 10, particularly on the edge
strength. Indeed, in order to achieve the very high edge strengths
of at least about 300 MPa to about 500 MPa or more, controlling one
or more of the processing parameters is important. The processing
parameters include: (i) an energy density of the laser beam 60,
(ii) a velocity of the laser beam 60 relative to the glass sheet 20
along the cutting line, (iii) a fluid flow of the cooling fluid 62,
and (iv) a minimum radius of curvature of the cutting line. The
discussion of the parameters, below, is based on experimentation on
Corning.RTM. EAGLE XG.RTM. alkali-free display glass. Although the
actual values of particular parameters may vary slightly with
different glass compositions, their manipulation will have similar
effects and results in other glass types, for example, other brands
of alkali-free display glass, display glass more generally, and
glass for other purposes.
With reference to FIG. 4, it has been found that the energy density
of the laser beam 60 has a significant effect on the edge quality
of the glass substrate 10. Among the possible edge quality
characteristics to examine, the relationship between the energy
density of the laser beam 60 and compression hackle was examined
closely. FIG. 4 is a plot showing compression hackle depth (in
micrometers, or microns, hereinafter "um") along the Y-axis and
energy density (J/mm) of the laser beam 60 along the X-axis.
Compression hackle is caused during the separation process and is
directly related to the amount of buckle or warp in the glass sheet
20 from the energy applied from the laser beam 60. The energy of
the laser beam 60 applies stress to the ultra-thin glass sheet 20,
which produces a warp in the glass sheet 20 (exhibiting sinusoidal
characteristics). As discussed above, crack propagation requires a
tensile stress in the direction of propagation through the
thickness of the glass sheet 20 from the bottom to the top. This
phenomenon utilizes tensile stress on the bottom surface of the
glass sheet 20 to separate or cut the part, but also applies a
compressive stress to the top surface of the glass sheet 20,
thereby creating compression hackle. The more stress applied, via
laser energy, the more compression hackle is exhibited. Therefore,
there is a balance between the laser energy applied and resultant
edge quality.
Control of the laser energy applied is dependent on both the laser
power produced by the laser beam 60 and the velocity of the laser
beam 60 relative to the glass sheet 20 along the cutting line. The
relationship is referred to as the energy density. Experimentation
was performed on a number of samples to measure certain effects of
the energy density, using a Gaussian spot laser beam of 2.5 mm
diameter (fixed). The energy density was computed as follows:
Energy Density (J/mm=Laser Power (J/s)/Velocity (mm/s). The effect
of controlling the energy density of the laser beam 60 may be
appreciated from reviewing the plot in FIG. 4, were a lower energy
density 402 shows significantly less compressive hackle and less
variability in the hackle depth measurements as compared with a
higher energy density 404. It is believed that controlling and/or
optimizing the energy density should be performed first, prior to
other tuning efforts, in order to achieve the aforementioned edge
strength goals.
Based on the above, controlling the energy density of the laser
beam 60 should be rigorously considered in order to achieve the
very high edge strengths contemplated herein. For example, in
connection with the glass sheet 20 thicknesses contemplated herein,
the energy density should be controlled to be at least one of: (i)
less than about 0.015 J/mm; (ii) less than about 0.014 J/mm; (iii)
less than about 0.013 J/mm; (iv) less than about 0.012 J/mm; (v)
less than about 0.011 J/mm; (vi) less than about 0.010 J/mm; (vii)
less than about 0.009 J/mm; and (viii) less than about 0.008
J/mm.
As mentioned above, velocity of the laser beam 60 with respect to
the glass sheet 20 is an important parameter of the energy density
calculation (and thus in edge quality). The velocity parameter
should be tuned in order to manage the propagation of the crack
along the cutting line, to optimize the crack performance, and
ultimately to ensure good edge strength. Surges in the propagation
of the crack have been observed during experimentation and are
believed to cause arrests (such as compressive hackle) in the edges
and even failure in the cutting process. Such arrests are difficult
to manage and require rigorous experimentation to determine proper
limits of operation, such as velocity.
FIG. 5 is a plot showing the relationship of velocity to a number
of arrest anomalies (such as compressive hackle) that occur during
the cutting process. The plot is characterized by the number of
occurrences of the anomaly on the Y-axis and velocity (mm/min) of
the laser beam 60 on the X-axis. The experimentation was performed
at a constant energy density per iterative change in velocity by
adjusting the laser power. Arrests were seen in the data at
relatively high velocity. Crack surging was also observed during
such high velocities. The arrests are very undesirable as they
yield extremely weak edges and can cause a jog in the cutting path
that is outside of the desired dimensional tolerances.
Based on the above, controlling the velocity of the laser beam 60
relative to the glass sheet 20 should be rigorously considered in
order to achieve the very high edge strengths contemplated herein.
For example, in connection with the glass sheet 20 thicknesses
contemplated herein, the velocity of the laser beam 60 relative to
the glass sheet 20 along the cutting line should be controlled to
be at least one of: (i) less than about 2000 mm/min; (ii) less than
about 1900 mm/min; (iii) less than about 1800 mm/min; (iv) less
than about 1700 mm/min; (v) less than about 1600 mm/min; (vi) less
than about 1500 mm/min; (vii) less than about 1400 mm/min; (viii)
less than about 1300 mm/min; (ix) less than about 1200 mm/min; (x)
less than about 1100 mm/min; and (xi) less than about 1000
mm/min.
As mentioned above, the effect of the cooling fluid 62 flow on the
edge strength is also an important consideration. In this regard,
reference is made to FIGS. 6 and 7. FIG. 6 is a plot of the
relationship between the flow rate of the cooling fluid 62 and the
resultant compressive hackle. The compressive hackle depth (um) is
shown along the Y-axis and the flow rate (liters per minute--lpm)
is shown on the X-axis. Hackle depth is measured with an optical
microscope at 20.times. magnification and is defined as the
distance from the top surface (laser incidence) of the glass to the
point at which the stress fracture line extends. The units are
typically defined as a direct measurement in microns but can be
defined as a percentage of the substrate thickness as well. FIG. 7
is a localized side view of the effects of the cooling fluid 62
applied from above the glass sheet 20 and the air bearing fluid
flow (and possibly vacuum) applied from below the glass sheet
20.
The nozzle 70 provides a dual role: (i) to apply the cooling fluid
62 in a precise orientation and location with respect to the laser
beam 60 (to locally cool the glass sheet 20, to exacerbate local
stress, and to propagate the crack); and (ii) to apply a mechanical
force to the top surface of the glass sheet 20 (to oppose the
buckling of the glass sheet 20 caused by the heat from the laser
beam 60). As shown in FIG. 7, the cooling fluid 62 flow may
interact in such a way as to affect localized influences on the
deformation of the glass sheet 20. For example, controlling the
flow rate of the cooling fluid 62 from above the glass sheet 20 may
provide: (i) some opposition to the buckling of the glass sheet 20
in the cutting zone (at relatively lower flow rates), (ii) relative
flattening of the glass sheet 20 in the cutting zone (at higher
flow rates), and/or (iii) a depression zone 120 (at even higher
flow rates).
The dual functions (cooling and mechanical pressure) may permit
further fine tuning of the cutting process in order to reduce
compression hackle and increase edge strength. For example,
increasing the flow rate of the cooling fluid 62 may decrease the
compressive stress, which aids in creating a high quality, high
strength edge. Note that the position of the nozzle 70 relative to
the laser beam 60 is dependent upon the glass sheet 20 thickness
and composition, which is important in applying the cooling and the
mechanical force in the precise location to maximize separation and
manage the crack behavior during propagation.
Based on the above, controlling the flow rate of the cooling fluid
62 should be rigorously considered in order to achieve the very
high edge strengths contemplated herein. For example, in connection
with the glass sheet 20 thicknesses contemplated herein, the fluid
flow of the cooling fluid 62 should be controlled to be at least
one of: (i) at least about 60 lpm; (ii) at least about 70 lpm;
(iii) at least about 80 lpm; (iv) at least about 90 lpm; (v) at
least about 100 lpm; (vi) at least about 110 lpm; (vii) at least
about 120 lpm; (viii) at least about 130 lpm; and (ix) at least
about 140 lpm. The foregoing cooling fluid rates are for air as a
cooling fluid, and the actual numerical value of the flow rates for
other cooling fluids may be slightly different based on their
specific heat capacity {in terms of heat flux/(mass times change in
temperature) or C=Q (m.DELTA.T), wherein C is specific heat
capacity, Q is heat flux, m is mass, and .DELTA.T is change in
temperature} relative to air. For example, if a cooling fluid had a
higher specific heat capacity than air, a smaller quantity of that
fluid, as by using a smaller flow rate for example, would be
necessary to achieve the same change in temperature of the glass to
create a similar glass-cutting stress profile as that created by
the air at the given flow rate. Beneficially, though, air can be
used to minimize stain on the glass, which may result from liquid
cooling. When cutting a free form line, the type of nozzle used
(and flow rate) differ from those for straight-line-only cutting.
For cutting free form lines, an annular nozzle is advantageous and
the air flow for any curved portions advantageously differs from
that for cutting straight line portions.
As mentioned above, the effect of the radius of any curved portion
of the cutting line on the edge strength is also an important
consideration. In this regard, reference is made to FIG. 8, which
is a plot illustrating that a radius of the cutting line has a
significant impact on edge quality and process performance. The
plot is characterized by compressive hackle depth (um) on the
Y-axis and radius (mm) on the X-axis. Radius cutting is a dynamic
process which requires additional energy and a more limited
velocity due to the nature of changing directions and cutting
through variable stress fields that are normally insignificant
during straight line cutting processes. The data of FIG. 8 shows
that the smaller the radius of curvature of the cutting line, the
higher the expected compression hackle depth. The process of
determining the cutting parameters (energy density, velocity, and
cooling fluid flow) for a radius dimension is similar as for a
straight line cut; however the particular set points may be
significantly different, and much more sensitive, particularly as
the radius becomes relatively small.
Based on the above, controlling the radius of curvature for a given
portion of the cutting line should be rigorously considered in
order to achieve the very high edge strengths contemplated herein.
For example, in connection with the glass sheet 20 thicknesses
contemplated herein, the minimum radius of curvature of the cutting
line is controlled to be at least one of: (i) at least about 2 mm;
(ii) at least about 3 mm; (iii) at least about 4 mm; and (iv) at
least about 5 mm.
A number of experiments were conducted on glass sheets 20 within
the thicknesses contemplated herein (e.g., 100 um) and using the
parameters discussed above. The edge strength of the resulting
glass substrates 10 were measured and plotted in FIG. 9. The plot
is characterized by failure probability (%) along the Y-axis and
maximum stress (MPa) along the X-axis. Thus, the edge strength is
expressed as the B10 value of the Weibull distribution (meaning
that with 95% confidence, 90% of the parts generated with an
optimized laser process will exceed the B10 strength measure). As
one can see, a statistically significant number of samples
exhibited edge strength in the 450-500 MPa range. For each of the
four data sets (triangle, diamond, circle, and square), the B10
value was greater than 300 MPa. For each of the circle and square
data sets, the B10 number is greater than or equal to 500 MPa. The
samples also exhibited minimal edge defects (e.g., chipping,
arrests and compression hackle). By comparison, FIG. 10 is a plot
for a mechanical cutting process (score and snap), wherein there
were used glass and shape parameters similar to those described
above in connection with FIG. 9. As seen from FIG. 10, the
mechanical cutting process yielded a B10 edge strength value of
only about 225 MPa.
Although the disclosure herein has been described with reference to
particular embodiments, it is to be understood that these
embodiments are merely illustrative of the principles and
applications of the embodiments herein. It is therefore to be
understood that numerous modifications may be made to the
illustrative embodiments and that other arrangements may be devised
without departing from the spirit and scope of the present
application.
* * * * *